FoldHelper™-ER3 is a recombinant baculovirus which provides for the expression of Aequorea victoria green fluorescent protein (GFP), (Acc# AAA58246) and E.coli beta-galactosidase (Acc# AAA24053) reporters, human protein disulfide isomerase (PDI), (Acc# CAA28775), calreticulin (Acc# AAH20493) and cyclophilin B (foldase) -a peptidyl-prolyl isomerase with a signal sequence, (Acc# AAH08848), (Price E.R. et al., Proc. Natl. Acad. Sci. USA, 88:1903-1907, 1991).
Calreticulin is a major chaperone facilitating glycoprotein folding. Co-expression of human calreticulin with lipoprotein lipase in insect cells resulted in a 9-fold increase in its enzymatic activity (Zhang L. et al., J. Biol. Chem., epub ahead of print, May 9, 2003). Glycoproteins often contain cysteine residues linked by disulfide bonds. PDI is a major chaperone participating in the formation of disulfide bonds.
PDIs have multiple functions and, though most abundant in the ER, they are ubiquitous (Turano, C., et al., J. Cell. Physiol., 193: 154-163, 2002). Initially, PDI was demonstrated to be efficient in oxidative protein folding in vitro (Weissman, J.S. and Kim P.S., Nature, 365: 185-188, 1993). Subsequently, overexpression of PDI was reported to improve yields of soluble or secreted protein in yeast and insect expression systems (Robinson A.S. et al., Bio/Technology, 12: 381-384, 1994; Schultz L. et al., Ann. NY Acad. Sci., 721: 148-157, 1994; Shusta, E.V., et al., Nature Biotechnology, 16: 773-777, 1998; Hsu T.A. et al., Protein Expr. Purif., 7: 281-288, 1996; Wang L., et al., J. Biol. Chem., 272: 27644-27651, 1997).
Peptidyl-prolyl cis-trans isomerases (PPIs) catalyze cis-trans isomerization of imide bonds, which can be a limiting step in folding of some proteins (Huang G.C. & Zhou J.M., J. Protein Chem., 19: 285-289, 2000). In addition, PPI can inhibit protein aggregation and exhibit chaperone-like activity, which is independent of its PPI activity (Ou W.B., et al., Protein Sci., 10: 2346-2353, 2001). PPI can cooperate with other chaperones in protein folding (Gothel S.F. & Marahiel M.A., Cell. Mol. Life Sci., 55: 426-436, 1999), in particular with PDI (Horibe T. et al., J. Biochem., 132: 401-407, 2002; Schiene C. & Fischer G., Curr. Opin. Struct. Biol., 10: 40-45, 2000; Altamirano N.M. et al., Proc. Natl. Acad. Sci. USA, 98: 3288-3293, 2001). In addition to PPI and chaperone activities, some PPIs (cyclophilins) also exhibit nuclease activity, and are likely to participate in apoptosis (Montague J.W. et al., J. Biol. Chem., 272: 6677-6684, 1997).
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